The present invention relates to a method for measuring the conversion efficiency of a catalyst and detecting deterioration thereof with air/fuel ratio sensors. More particularly, the invention pertains to a method for measuring mean converted rates of hydrocarbons+carbon monoxide+nitrogen oxides (HC/CO/NOx), NOx, and HC/CO. The invention is also utilized for detecting deterioration of a catalyst so as to alert the operator to replace the catalyst in a timely manner.
A conventional method detects deterioration of a catalyst based on the difference between the maximum output voltages of two oxygen sensors disposed on the upstream and downstream sides of the catalyst (Japanese Published Unexamined Patent Application No. Sho-63-231252). An air/fuel ratio controller has also been disclosed, which adequately controls the air/fuel ratio with two/fuel ratio sensors even with a deteriorating catalyst (Japanese Published Unexamined Patent Application No. Sho-63-205441).
The output characteristic of an oxygen sensor changes abruptly around .lambda.=1. In other words, as shown in FIG. 12, the waveform of the output switches between a lean air/fuel ratio and a rich air/fuel ratio with a steep slope and does not show an intermediate or mean value. Accordingly, in the conventional method for detecting deterioration of a catalyst with two oxygen sensors, processing of comparison data between two waveforms is rather troublesome and complicated. Moreover, accurate and precise detection is difficult for a converted rate of lower than 70% as seen in FIG. 13.
Although analyzers are useful for directly determining the concentration of each component of exhaust gas, i.e., HC, CO, and NOx, (especially useful for the determination of NOx), they are composed of numerous devices, large-sized and expensive; in addition, the measurement process is quite complex.